1506
J . Am. Chem. SOC.1983, 105, 1506-1509
G17 for (CH3)2N, where L = 117°.22 Another aspect is the decrease +I a(14N)that occurs on replacing CF3 or CF3S groups in (CF3S),N by phenyl groups (Table II).23 The decrease reflects delocalization of the unpaired spin into the phenyl system. The increase in g factor in going from (C6H5)?N to (C,H$)2N probably results from the larger spin-orbit coupling due to the S From molecular models the following W-shaped structure is suggested for N(SCF3)2. For (CH3)(CH30)Nthis type of
0
4.
..CF2
structure has also been suggested, based on INDO calculations.20 The C-S-N-S-C skeleton together with the lone electron pair are assumed to be approximately in the nodal plane of the gorbital occupied by the unpaired electron. This is supported by the low and,l3CY.The S-N-S angle is assumed coupling constants for % to be > K,), Co(I1) porphyrins are characterized by a low affinity for the second axial ligand (K2
L
5
w
4
0
-J--.)
350
1
400
I
I
' ' 550
500
'
I
I
(nm)
A Figure 1. Initial difference spectrum following laser photolysis of (piperidine)Co"DPDME (+-) and calculated difference spectrum between (piperidine)Co"DPDME and CoIIDPDME (--E-). The spectra have been normalized for easier comparison.
Experimental Section Cobalt(II)4euteroporphyrin IX dimethyl ester (CoIIDPDME) was synthesized and purified according to the procedure of Caughey et al." Toluene and liquid ligands were redistilled before use. Toluene solutions of Co"DPDME (lo4 M ) were deaerated by bubbling argon. In the range of ligand concentrations used ( 5 X lo4 to 3 X M) at least one-third of the porphyrin was present as the pentacoordinated species. The apparatus and the techniques used in the laser photolysis experiments have been described in details in a previous paper.5 The photodissociation was triggered using the 530-nm harmonics of a Qswitched neodymium laser (pulse width = 20 ns). Transient absorbance changes were monitored at the maximum of the a-band of the tetracoordinated porphyrin near 550 nm. The minimum absorbance change which could be detected was =lo-' with a time constant of -50 ns. The transient spectrum and the quantum yields were obtained from the initial absorbance changes and corrected for the fluctuations of the laser output. Ferrohemochromes were used as a reference for the quantum yield mea~urements.~ All experiments were performed at 25 OC.
Results and Discussion Upon photolysis of the pentacoordinated base-CoIIDPDME complexes the absorption near 550 nm transiently increased. No signal was observed in the absence of ligand. The difference ~
~~
(2) Hoffman, B. M.; Gibson, Q.H. Proc. Natl. Acud. Sci. USA 1978, 75, 21-25. (3) Traylor, T. G.; White, D. K.; Campbell, D. H.; Berzenis, A. P.J . Am. Chem. SOC.1981, 103,4932-4936. (4) Momenteau, M.; Lavalette, D. J . A m . Chem. Soc. 1978, 100, 4322-4324. ( 5 ) Lavalette, D.; Tetreau, C.; Momenteau, M. J. Am. Chem. SOC.1979, 101. 5395-5401. ( 6 ) James, B. R. "The Porphyrins", Vol. V, Dolphin, D., Ed.; Academic Press: New York. 1978: DD 258-277. (7) Stynes, D. V.; Stynks, H. C.; James, B. R.; Ibers, J. A. J. Am. Chem. SOC.1973. 95. 1796-1801. (8) Walker, F. A. J. A m . Chem. SOC.1973, 95, 1150-1153. (9) Walker, F. A. J. A m . Chem. SOC.1973, 95, 1154-1159. (10) The photodissociation of cobalt-substituted myoglobins Co"MbN0 and Co"Mb0, is rewrted in ref 2. (11) Caughky, W'. S.; Alben, J-0.; Fujimoto, W. Y.;York, J. L. J . Org. Chem. 1966, 31, 2631-2640.
Figure 2. Recombination rates of L = 4-methylpyridine with CoIIDPDME after photolysis: porphyrin concentration, lo4 M in toluene; temperature, 25 OC.
spectrum recorded at t = 0 was found to reproduce reasonably well the static difference spectrum between pentacoordinated and free CoIIDPDME (Figure 1). It can therefore be concluded that photolysis simply removes the nitrogenous base of the pentacoordinated species. The photodissociation quantum yields were in the range 0.01-0.1, Le., of the same order of magnitude as those reported No systematic dependence on the previously for hemochrome~.~ base pKa could be noticed. Typical values were 0.03, 0.03, and 0.05 for 4-cyanopyridine, edimethylaminopyridine, and piperidine, respectively. The recombination reaction was exponential and the relaxation rate was a linear function of the concentration of ligand (L) (Figure 2). Provided that (L) >> (P), the relaxation rate of equilibrium 1 is given by k = k,(L) k-, (3)
+
1508 J. Am. Chem. SOC.,Vol. 105. No. 6, 1983
Tetreau. Lmalette. and Momenteau
I k
lo7
(s-l)
1
10
9
-
8 10
1 1 1 1 1 1 1 1 1 1 , , 5
0
10
6' -N-Me-N-
0
0
+
S
(i
L
-N'y'N-
0 0
*-N-
Me-N-
I
+
-
Fen
(d6 1
co
II
(d7)
++ +4-k
-
d x w
+ -#+dzz
dXZ+
dYZ
[5=2J
-
-
+
+
The situation encountered with Co(I1) complexes was particularly favorable since, contrary to the hemochrome: k-l was of an order of magnitude comparable to kl (L); the association and dissociation rate constants could thus be obtained directly from the slope and intercept of the linear plots of k against (L) (Figure 2). Their ratio yielded the equilibrium constant K l (q1). The results are presented graphically in Figure 3 in which log k*l and log K1are plotted against the pK, of the reacting base. Similar correlations have been already reported for the equilibrium constant K1 and for the enthalpy of ligand binding to various Co(II)-p~rphyrins.'*~ As in the case of hemochromes,5 the kinetic data give a still better insight into the factors which govern the reactivity ("on"-rates) or the intrinsic stability ("off"-rates). For the sake of comparison, our previous results for the binding of the second axial base in hemochromes (reaction 2) are also shown in dotted lines in Figure 3. Remembering that pentacoordinated cobalt(I1) and hexacoordinated iron( 11) porphyrins respectively follow reaction
+-+k
-%-
dXY
[ T I
+ k +
10
++ ++ ISr1/2]
-T-
#+ -+ ?I[
schemes 1 and 2, their differences and similarities are more conveniently discussed in terms of the metal orbital occupancy diagram displayed in Figure 4. Although the porphyrin and the metal atom differ in (L)Fe"TPP-B hemochromesI2 and in the present (L)Co"DPDME complexes, the situation immediately following photodissociation is similar: one ligand molecule binds to a complex in which the dz2 metal orbital is occupied by a single electron (respectively the high-spin pentacoordinated Fe"TPP-B complex and the tetracoordinated C0"DPDME). However, the position of the metal atom with respect to the porphyrin plane is opposite in the reactants and in the products. The cobalt atom, which is in the plane in the reacting tetracoordinated CoI'DPDME, is displaced out of plane in the final pentacoordinated (L)CoIIDPDME; on the contrary, the iron atom is out of plane in the initial pentacoordinated (L)Fe"TPP but becomes coplanar (12) Abbreviation: (L) denotes an external ligand, and -B a covalently linked base, as defined in ref 5 .
Laser Photodissociation of Nitrogenous Bases
J. Am. Chem. Sot., Vol. 105, No. 6, 1983 1509
in the Anal hexacoordinated hemochrome (L)Fe"TPP-B (Figure 4). The pKa dependence of the binding constant of hemochromes (k,) has been attributed in part to the repulsion at long distance between the base dipole moment and the dipole of the pentacoordinated porphyrin resulting from the out of plane displacement of the iron.5 As the base dipole moment varies linearly with pKa, this repulsion term in the energy bamer for reaching the transition state increases with the ligand basicity. In this respect, a remarkable exception is piperidine, which has a small dipole moment in spite of its high pKa and which accordingly reacts faster with iron(1I) than expected and deviates significantly from the correlation (Figure 3a). As tetracoordinated Co(I1) porphyrins are expected to be devoid of dipole moment, this repulsion term should vanish. Indeed the pK, dependence of kl in the cobalt complexes is much less pronounced than that of k2 in hemochromes. The fact that piperidine now also follows the same correlation as other ligands further confirms the role of dipole repulsion terms in hemochromes and their absence in the cobalt system (Figure 3a). In the region of low pKa (where the dipole repulsion term is the smallest) cobalt(I1) kl and iron(1I) k2 values approach each other and are close to diffusion control; thus the reactivity of both systems is not very different, but for the additional dipole-dipole energy barrier in the hemochromes. This might seem surprisin in view of the fact that the iron atom is displaced roughly 0.5 out of the porphyrin plane in the pentacoordinated Fe(I1) comp l e ~ e s , while '~ the cobalt atom remains virtually coplanar with the macrocycle in CoIIDPDME (Figure 4). However, the critical Fe-N(base) distance in the transition state can be as high as 5-7 and the field experienced at long distance by the approaching ligand might not be very sensitive to the small displacement of the metal atom. The direct repulsion between the lone pair electrons of the base and the single dz*metal electron might then constitute the leading part of the energy barrier and account for the residual pKa dependence observed in the cobalt complexes. The pKa dependence observed for k-, and K, indicates that the stability of the final complexes increases with the a-donor strength of the ligand (Figures 3b and 3c). The lesser affinity of ligands toward tetracoordinated C0"DPDME as compared to penta-
x
(13) Collman, J. P. Acc. Chem. Res. 1977, 10, 265-272.
coordinated Fe"TPP-B (Figure 3c) can be almost entirely attributed to a better stabilization of the final hexacoordinated iron complex. The dissociation rates k-l for (L)Co"DPDME are about two orders of magnitude higher than the k-, rates in hemochromes (Figure 3b). The greater stability of hemochromes has been rationalized in terms of occupation of the metal orbital^;^ after going to the final low-spin hexacoordinated complex, the d,l orbital becomes vacant, while it remains occupied and destabilized on formation of the pentacoordinated cobalt complex (Figure 4). This destabilization has been invoked to account for the small values of Kz in cobalt' and is likely to be at the origin of the high dissociation rates as well. Another consequence is a longer bond in (L)Co" porphyrins as distance (Co-N(base) = 2.16 compared to (L)Fe" porphyrin-base (Fe-N(base) = 2.02 Furthermore, the cobalt atom is slightly displaced by 0.14 8,above the porphyrin. As the "off" rates are predominantly governed by short-distance interactions, the longer bond is relatively favorable for sterically hindered ligands such as piperidine. This ligand was found previously to deviate from the k-, vs. pKa correlation presumably because of the interaction of its hydrogen atoms with the porphyrin.' Due to the reduced interaction in the cobalt complexes, piperidine no longer remains an exception (Figure 3b). It is the strongest ligand because it is the best u donor. Another consequence of the longer cobalt-base bond is the reduction of eventual R interactions. This is also shown in Figure 3b, where 4-cyano- and 4-acetylpyridine now follow the cobalt correlation, contrary to the situation in hemochromes where additional charge transfer at short distance is probably at the origin of their greater stability as compared to other ligands.' In short, both the longer metal-N(base) distance and the absence of exceptions in the k-I vs. pKa correlation point to minimal (if any) contribution of R bonding in pentacoordinated complexes of CoIIDPDME. Registry No. (L)Co"DPDME (L = 4-cyanopyridine), 84081-86-7; (L)Co"DPDME (L = 4-acetylpyridine), 8408 1-87-8; (L)Co"DPDME (L = 4-methylpyridine), 84081-88-9; (L)Co"DPDME (L = 4-dimethylaminopyridine), 84081-89-0; (L)Co"DPDME ( L = pyridine), 29130-56-1; (L)Co"DPDME (L = 1-methylimidazole), 84081-90-3; (L)Co"DPDME (L = piperidine), 8408 1-91-4; CoIIDPDME, 1589211-2. (14) Scheidt, W. R. J . A m . Chem. SOC.1974, 96, 90-94. (15) Jameson, G. B.; Ibers, J. A. Inorg. Chem. 1979, 18, 1200-1208.
